1. Field of the Invention
This invention relates to a process for improving start up and steady rate friction of soft/compliant polyurethanes and to polyurethane elements which have been treated by the process. An important potential application for such treated material is in compliant layer bearings, for example, those used in artificial joint replacements, although it also has other applications.
2. Description of the Prior Art
Typically these artificial joint replacement bearings are relatively recent and rely on the use of soft/compliant polyurethanes (or other elastomeric materials) to improve the lubrication mechanism compared with convention artificial joints. The concept of using a compliant polyurethane layer on the bearing surface of an artificial joint replacement is inspired by the natural synovial joint which has such a compliant layer, the articular cartilage. A combination of lubrication mechanisms have been proposed for synovial joints which result in fluid film lubrication, where the joint surfaces are completely separated by a thin film of lubricant. Currently most artificial joints are based upon metal on ultra high molecular weight polyethylene (UHMWPE), ceramic on UHMWPE or metal on metal material couples. These bearings are far less compliant than the natural joint and hence operate in a mixed lubrication regime, with partial contact of the two bearing surfaces. This leads to higher friction, and wear of the bearing surfaces of conventional joints. In contrast, compliant bearings operate with fluid film lubrication and extremely low friction, and hence potentially negligible wear and a long implant life.
The design parameters for the construction of compliant layer bearings are known. Polyurethane elastomers have been the materials of choice for these bearings. Under conditions of cyclic loading and motion typical of the major load bearing joints in the human body there is much experimental evidence that this type of bearing will operate with extremely low friction, typical of fluid film lubrication. In contrast, Caravia et al (L. Caravia, D. Dowson and Fisher J., Start up and steady rate friction of thin polyurethane layers. Wear, (1993), 160, 191-197) cites that the use of compliant polyurethane layers in this application can result in unacceptably high friction values under conditions which combine heavy loading and low sliding velocities, i.e., at the onset of motion (start up friction).